Metal-azo and metal-azomethine dyes, having a single dye ligand complexed to a metal, are known in the art (see, for example, Drew, H. D. K.; Fairbairn, R. E. J. Chem. Soc. 1939, 823-35; Beech, W. F.; Drew, H. D. K. J. Chem. Soc. 1940, 608-12; Steiner, E.; Mayer, C.; Schetty, G. Helv. Chem. Acta 1976, 59, 364-76; U.S. Pat. Nos. 4,012,369; 4,123,429; and 4,265,811). These 1:1 complexes are predominantly used in two applications; color photography (e.g., see U.S. Pat. Nos. 3,453,107; 3,551,406; 3,544,545; 3,563,739; 3,597,200; 3,705,184; 3,752,836; 3,970,616; 4,150,018; and 4,562,139), and the dyeing of textiles (e.g., see U.S. Pat. Nos. 3,878,158; 4,045,423; 4,218,367; and 4,617,382; and European Pat. No. 144776). The 1:1 complexes have a central metal ion coordinated to a dye and additional ligands (e.g., water, pyridine, ammonia, or ethanolamine).
In cases where the additional ligand is an inorganic species (e.g., water or ammonia), the physical properties are much like other coordination complexes in that they possess high melting points and poor solubility in organic media. Even when the additional ligand is a small organic molecule, such as pyridine, the physical properties of the resulting metal-azo dye complex are often not like most organic substances. A typical example is [2,2-azobis[phenolato](-2)-N,O,O,](pyridine)copper which is reported to decompose at 310.degree. C., apparently without melting. Reinvestigation of this compound using thermogravimetric analysis shows that the pyridine is in fact lost in the 100.degree.-160.degree. C. region.
Aside from the very early work of Drew et. al., melting point information is very sparse. In the Colour Index, the only melting point information on identifiable metal-azo dyes is on acid-basic complexes which generally consist of an anionic metal-azo complex and a positively charged organic dye. There have been several papers where melting temperatures are given for the unmetallized dye, but no information is given for the corresponding metal complexes.
There are also 1:1 metal-azo complexes containing 2- and 6-substituted pyridines, which form polydentate chelates that have been disclosed. For example, U.S. Pat. No. 4,562,139 discloses metal-azo dye complexes containing tridentate nitrogen-containing heterocycles. These materials are used in various photographic constructions and were designed to be compatible with gelatin. In the examples, the materials were coated from either aqueous or methanolic media and it is unlikely that they would be oil soluble dyes.
A survey of all the solvent dyes identified as either metal-azo or acid-basic dye complexes in the Third Edition, and subsequent supplements, of the Colour Index reveals that very few metal-azo dyes possess significant solubility in hydrocarbon media. Of the 152 dyes identified as metal-azo dyes or acid-basic dye complexes, only sixty-six had any solubility data for hydrocarbon solvents (turpentine, white spirits and mineral oil); sixty are listed as insoluble to very slightly soluble; five are listed as slightly soluble to soluble; and only one entry is listed as very soluble.
In allowed copending U.S. application Ser. No. 7/667,658, filed Mar. 11, 1991, metal-azo dyes containing pyridines containing free-radically polymerizable groups are disclosed. In addition to demonstrating that these materials could undergo free-radical polymerization reactions under standard conditions, it was found that in some instances, those materials also had good solubility in organic solvents.
One use of organic soluble metal-azo dyes is in thermal transfer printing. The term thermal printing covers two main technology areas. In thermal transfer printing of textiles, a donor sheet is coated with a pattern of one or more dyes; contacted with the fabric to be printed; and heat is uniformly administered, sometimes with concomitant application of a vacuum. The transfer process has been much studied, and it is generally accepted that the dyes are transferred by sublimation in the vapor phase. Pertinent references include: Bent, C. J. J. Soc. Dyers Colour. 1969, 85, 606; Griffiths, J.; Jones, F. Ibid. 1977, 93, 176; Aihara J. Am. Dyest. Rep. 1975, 64, 46; Vellins, C. E. In The Chemistry of Synthetic Dyes; Venkataraman, K., Ed.; Academic Press: New York, 1978; Vol. 8, p. 191.
The other area of thermal printing is thermal imaging, where heat is applied in an image-wise fashion to a donor sheet in contact with a suitable receptor sheet to form a colored image on the receptor. In one embodiment, termed thermal mass transfer printing, as described for instance in U.S. Pat. No. 3,898,086, the donor is a colorant dispersed in a wax-containing coating. Upon the application of heat, the construction melts or is softened and a portion of the colored donor coating transfers to the receptor. Despite problems with transparency, pigments are generally the colorants of choice in order to provide sufficient light fastness of the colored image on the receptor.
Another means of thermal imaging is termed variously thermal transfer imaging or recording, or dye diffusion thermal transfer. In this case, the donor sheet comprises a dye in a binder. On image-wise application of heat, the dye, but not the binder, is transferred to the receptor sheet. A recent review has described the transfer mechanism as a "melt state" diffusion process quite distinct from the sublimation attending textile printing (Gregory, P. Chem. Brit. 1989, 25, 47). This same review emphasizes the great difficulty of developing dyes suitable for diffusive thermal transfer. With regard to the available conventional dyes, it was stated that ". . . It is significant that of the one million or so dyes available in the world, none of them were fully satisfactory . . . " Among the failings of these dyes are inadequate light and heat fastness of the image and insufficient solubility of the dyes for coating in the donor sheet. As has been noted previously, light fastness is also a problem in mass transfer imaging systems. In fact, achieving adequate light fastness is probably the single most important challenge in these constructions. In large measure this is the result of the diffusive thermal transfer dye image being a surface coating a few microns thick. The dye is thus readily susceptible to degradation by photo-oxidation. In contrast, textile fibers, which are 100 times thicker, are uniformly dyed throughout their depth, so that fade in the first few microns at the surface is of little practical importance. In consequence, it is common to find that dyes showing good light fastness in textile printing exhibit very poor photostability in the diffusive thermal imaging (see, for example, U.S. Pat. No. 4,808,568). There remains, therefore, a strong need for improved dyes for this latter application.
There is very little known about the use of metal-azo dyes in thermal printing art. A review on transfer printing (Datye, K. V.; Vaidya, A. A. Chemical Processing of Synthetic Fibers and Blends; John Wiley & Sons: 1984, p. 407) states: "Acid and metal-complex dyes which are commonly used for dyeing nylon are unsuitable for heat-transfer printing because these dyes have high melting points and low vapor pressures and hence, do not get vaporized and transferred below 200.degree. C. However, the recently developed Dew Print.TM. machine enables wet-transfer printing of the acid and metal-complex dyes on nylon." The wet-transfer-process dyes of the above article require the presence of water solubilizing groups such as sulfo and carboxy, and the dyes are generally charged. This process involves the dissolution of the dye in water and transfer to the substrate. Further details of this process are given in U.S. Pat. No. 4,155,707.
Metal-azo dyes have been used in mass transfer printing. In Japanese Kokai Patent Application No. 62-021594, it is stated that "the ink layer is completely transferred to plain paper when the transfer recorder is peeled from plain paper"-- an indication that both the binder and the colorant are transferred. Moreover, the binders used in the practical examples are all low molecular weight (less than 2000 Daltons), except for the control which was demonstrated to not transfer efficiently. The colorants used were high melting pigments, some of which were calcium or sodium salts of azo dyes. These salts are ionic in nature and are generally not soluble in organic solvents. In a related case, Japanese Kokai No. 62-021593, the process being discussed is also mass transfer, however, the colorants were "oil soluble". Some of these oil soluble dyes were metal-azo dyes wherein the structures were not explicitly disclosed. The metal-azo dyes that could be identified were found to be negative charged 2:1 (metal:azo) complexes. The solubility characteristics of the dyes, for which structures were not available, indicate that they are probably 2:1 complexes as well.
Other embodiments of mass transfer system utilizing metal-azo dyes are discussed in U.S. Pat. Nos. 4,585,688, 4,664,670, and 4,784,905. Described in U.S. Pat. No. 4,585,688 is a transfer medium comprised of a heat-resistive support, a colorant layer containing a binder and a coloring agent (which may be a metal-azo dye), and a transferable layer comprising a low molecular weight compound capable of containing a coloring agent and transferring an image to a paper receptor. In U.S. Pat No. 4,664,670, a thermal transfer donor construction requiring the presence of a low melting, essentially colorless, non-polymeric, organic nitrogen-containing, impregnating reagent for the printing of textiles is disclosed. A thermosensitive image transfer recording medium comprised of a support material and a thermofusible ink layer is described in U.S. Pat. No. 4,784,905. The thermofusible ink layer contains a fine porous resin structure made of a resin containing: (1) a coloring agent (which may be a metal-azo or metal-azomethine dye); and (2) a carrier material (for holding the coloring agent at normal temperatures and also for carrying the coloring agent out of the thermofusible ink layer for image formation upon application of heat), and (3) an image gradation control agent.
There are also several published patent applications (see, for example: Japanese Kokai Nos. 63-144084, 60-2398, and 59-78893) which disclose the use of metallizable azo dyes in thermal transfer donor constructions. In these cases, the donor layer comprises a binder and an azo dye that is capable of chelating to a metal. The azo dye is thermally transferred to a receptor layer which contains a metal salt which can react with the azo dye. The generation of a metal-azo dye by this method has several potential drawbacks because (1) the colors of the azo dyes and the metallized dye are different, the resultant color being dependent on the extent of metallization; (2) metallized dyes are generally much more resistant to light induced fade and therefore, if both azo dye and metallized-azo dye are present the color may change as a function of light exposure; and (3) the chelation of the azo dye to a metal often involves the generation of acid which could have a deleterious effect on image stability. This problem can be overcome by addition of buffering agents, however, this further complicates the donor or the receptor formulation.
The utility of some the materials of the present invention for thermal dye diffusion transfer is generally described in allowed co-pending U.S. application Ser. No. 7/667,323, filed Mar. 11, 1991. Aside from the work in this copending application, the foregoing art does not disclose or teach the preparation and use of oil-soluble metal-azo dye complexes having ancillary ligands, free of free-radically polymerizable groups that can be conveniently prepared.